Referring to FIG. 1, a typical gas turbine engine 10 includes an engine casing 12 that houses a compressor rotor 14, a combustor 16, a turbine rotor 18, and a shaft 20 connecting the compressor rotor to the turbine rotor. The shaft is rotatably mounted to the casing by way of an internal gearbox 22 and bearings 24 disposed between the compressor rotor and the turbine rotor. The compressor rotor and the turbine rotor rotate with the shaft. The gas turbine engine is mounted within a nacelle 25. The gas turbine engine also includes various accessory components that receive mechanical power from an accessory gearbox 26 that is mounted external to the engine casing. The accessory gearbox receives mechanical power from the gas turbine engine shaft via a power-takeoff shaftline 28 that extends radially outward from the internal gearbox through the gas turbine engine casing, substantially as disclosed, for example, in U.S. Pat. No. 7,246,495 issued to Muramutsu et al.
The compressor rotor 14 includes an axially-spaced plurality of compressor discs 30 mounted to the shaft and separated by interstage spacers 32 also mounted to the shaft. The compressor rotor discs and the interstage spacers together form the compressor rotor. Each compressor disc supports a circumferential array of compressor blades 34, while each interstage spacer supports a series of labyrinth seal teeth. The design of the compressor as a whole is driven by working fluid requirements of the gas turbine engine thermodynamic cycle. Additionally, the compressor rotor assembly is optimized for weight reduction, key dimensions of the discs and the blades being manufactured to approximately 0.0001″ tolerances in order to have just enough material to support centripetal strains induced when the rotor rotates at operational speeds under operational temperatures. Extensive efforts are made to eliminate stress risers such as holes, corners, or sharp indentations in the surface or the cross-section of the compressor disc and of each compressor blade. Special effort is given to optimizing the blade root region, where each compressor blade is mounted to a compressor disc. Special materials also are chosen for use in the compressor rotor. Typically, titanium is preferred for its strength and toughness at elevated temperatures. The turbine rotor 18 is similarly constructed.
Adjacent to the compressor rotor, the gas turbine engine casing supports a plurality of compressor vane assemblies 36 that are axially interspaced with the circumferential arrays of compressor blades. Each compressor vane assembly includes a circumferential array of vanes 37 that extend inward from the casing to a shroud ring 38 that is closely radially adjacent to a corresponding interstage spacer of the compressor rotor. Typically, each vane is pivotable around a shaft extending radially from the casing to the shroud ring, and each circumferential array of vanes is synchronously movable by a control ring mounted outside the gas turbine engine casing. Vane dimensions are optimized for aerodynamic performance (low flow resistance) and for maximal variability of flow area as the vanes are rotated. The turbine portion of the casing is similarly constructed.
Referring to FIG. 2, the accessory gearbox 26 houses an accessory geartrain 40 for driving the accessory components. The accessory components include, for example, a fuel pump 42, an oil pump 44, and an alternator or generator 46. Each accessory component is mounted directly to the accessory gearbox and is driven from a corresponding gear within the accessory geartrain. The accessory gearbox also supports an engine starter 48, which can be pneumatic, hydraulic, electrical, or combustion-driven. The engine starter is clutch-connected to the accessory geartrain so that the engine starter will not place an unnecessary load on the accessory geartrain during operation of the gas turbine engine 10. Optionally, the engine starter can be integrated into the accessory gearbox, as disclosed, for example, by U.S. Pat. No. 5,555,722 issued to Mehr-Ayin et al.
During engine startup, the engine starter drives the power-takeoff shaftline 28, via the accessory geartrain, in order to turn the shaft 20 of the gas turbine engine 10. Thus, the typical accessory gearbox transfers power from the power-takeoff shaftline to the accessory components during operation, and provides power to the power-takeoff shaftline from the engine starter at startup. For weight reduction, the engine starter can be integrated with the generator to provide a combined starter/generator.
Referring to FIG. 3, a typical combined starter/generator 49 includes a stator 50 housing primary windings 54 and exciter field windings 55, and also includes a rotor 60 housing rotor windings 62 and exciter windings 65 that are electrically connected with the primary field windings. As will be apparent to those of ordinary skill, the rotor is housed within the stator for rotation about an axis of the stator, and the different windings of the starter/generator can be selectively energized for operation as a starter (motor) or as a generator.
For operation of the combined starter/generator 49 as a generator, the exciter field windings 55 are energized to produce a (typically stationary) exciting magnetic field, and tangential motion of the exciter windings 65 across the exciting field induces electrical current in the exciter windings and in the rotor windings 62. In turn, tangential movement of the rotor windings across the primary windings 54 induces electrical currents in the primary windings. For operation as an asynchronous starter motor, the primary windings are energized to produce a rotating magnetic field, which electromechanically couples with the rotor windings to cause rotation of the rotor. For optimal performance of the combined starter/generator, the electromagnetic gaps 67 between the rotor windings and the primary windings, and between the exciter field windings and the exciter windings, are kept as small as possible. Typically, the rotor windings 62 and the exciter windings 65 are at the circumference of the rotor 60 and are separated from the primary windings 55 by only a narrow (<0.2 inch) air gap to allow for thermal and mechanical strains at operating speed and temperatures.
Since the starting torque for the gas turbine engine 10 is significantly larger than the torque required by any other mechanical load on the accessory geartrain 40, the gas turbine engine starting torque determines the sizes and weights of the internal gearbox 22, the power-takeoff shaftline 28, the accessory geartrain 40, and the starter 48. As can be seen from FIG. 1, the size and placement of the internal gearbox affect the overall axial and radial size of the gas turbine engine and of the gas turbine engine nacelle 25. Additionally, for gas turbine engines mounted on aircraft, the power-takeoff shaftline and the accessory gearbox 26 introduce structural asymmetries that can adversely impact the aerodynamic performance of the gas turbine engine nacelle. Thus, mechanical transfer of power to and from the gas turbine engine shaft, via the radial power-takeoff shaftline and the accessory geartrain, plays an undesirably important role at many stages of gas turbine engine design.
Accordingly, efforts have been made toward developing a more electric engine. For example, it has been proposed to incorporate an electric starter/generator machine into a shaftline of a gas turbine engine, in place of an internal gearbox. However, one-for-one replacement of the gearbox with the electric machine has proven challenging. Structural requirements and temperature limitations have limited implementation of the more electric engine concept. In particular, materials typically used in generators are excessively heavy, do not provide adequate mechanical strength, and/or present unacceptable dimensional variations at the operating temperatures and rotational speeds typically achieved within a gas turbine engine. Typical engine temperatures range from about 1300 F (for high-pressure cooling air) up to about 2800 F (at the combustor exhaust). By contrast, common electrical machine winding materials such as aluminum or copper liquefy at 1220 F or 1984 F, respectively. These materials also exhibit significant thermal strains throughout the operating temperature range of a typical gas turbine engine. Although permanent magnets can be arranged to provide satisfactory electromagnetic coupling with less weight, typical permanent magnet materials such as neodymium iron boron exhibit structural instability due to thermal fatigue, and also exhibit rapid demagnetization above the materials' respective Curie temperatures. Curie rapid temperatures for typical permanent magnet materials range from 590 F up to about 680 F, while long-term operating temperatures do not exceed 400 F.
Alternatively to replacing the internal gearbox, an electric machine can be integrated into fan casing struts and a fan rotor of a gas turbofan engine, as taught by Sharp in U.S. PG Pub. 20070157597, now abandoned. However, fan rotors preferably are designed to be as thin and light as possible for enhanced aerodynamic performance, so that the added weight and volume required for generator windings is not desirable in a fan of a gas turbofan engine. Additionally, fan rotor rotational speeds are limited by aerodynamic considerations such as angle-of-attack and drag, while it is desirable for a starter/generator to move at a rotational speed determined by electrical bus requirements.